Optical encoder

ABSTRACT

A light beam emitted from a semiconductor-laser light source is projected onto a diffraction-grating scale after passing through a collimator lens, a beam splitter and a central portion of an annular reflection grating. Two diffracted light beams reflected from the diffraction-grating scale are projected onto the annular reflection grating. The annular reflection grating diffracts the light beams projected onto all portions thereon to a substantially original direction to be projected onto and diffracted from the same position on the diffraction-grating scale. The diffracted light beams are superposed and the resultant light beam is returned to the beam splitter. The light beam is guided by the beam splitter in a direction different from the semiconductor-laser light source, and is detected by a photosensor as an interference light beam. Even if the oscillation wavelength of the semiconductor-laser light source changes, for example, due to a change in the temperature environment, to change the diffraction angles of the diffracted light beams, the light beams are diffracted with original diffraction angles by the annular reflection grating, the position of rediffraction by the diffraction-grating scale and the state of emitted light beams are invariable. Hence, the state of interference is stable.

This is a division of application Ser. No. 09/780,433, filed Feb. 12,2001 now U.S. Pat. No. 6,831,267.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical encoder having a reflectionmechanism using an annular diffraction grating for detecting informationrelating to the position or the angle in an industrial measuringapparatus or the like.

2. Description of the Related Art

FIG. 1 is a perspective view illustrating the configuration of aconventional linear encoder. In FIG. 1, a light beam from asemiconductor-laser light source 1, serving as a coherent light source,is divided into polarized components by a polarizing beam splitter 3 viaa collimator lens 2. A P-polarized light beam passing through thepolarizing beam splitter 3 is projected onto a diffraction-gratingportion on a scale 4 with an angle θ after being propagated on anoptical reflecting surface, is emitted as a +first-order diffractedlight beam by being reflected, is returned to the original optical pathby a reflecting optical element 6 via a ¼-wavelength plate 5, and isfinally returned to the polarizing beam splitter 3 by being subjected to+first-order diffraction.

An S-polarized light beam reflected by the polarizing beam splitter 3 isprojected onto the diffraction-grating portion on the scale 4 with anangle θ after being propagated on the optical reflecting surface, isemitted as a −first-order diffracted light beam by being reflected, isreturned to the original optical path by a reflecting optical element 6via a ¼-wavelength plate 5, and is returned to the polarizing beamsplitter 3 by being subjected to −first-order diffraction.

Since the ¼-wavelength plate 5 is inserted in the optical path of eachof the diffracted light beams, the orientation of polarization isshifted by 90 degrees during the back and forth movement, so that the+first-order diffracted light beam and the −first-order diffracted lightbeam are returned to the polarizing beam splitter 3 as an S-polarizedlight beam and a P-polarized light beam, respectively. Accordingly, the+first-order diffracted light beam is reflected by the polarizing beamsplitter 3 and the −first-order diffracted light beam passes through thepolarizing beam splitter 3, and the two light beams are emitted in astate in which the wave surfaces of the two light beams are superposed.Then, the superposed light beams are converted into a linearly-polarizedlight beam, in which the orientation of polarization changes based onthe phase difference between the two light beams, while passing througha ¼-wavelength plate 7. The light beam is then divided into two lightbeams by a non-polarizing beam splitter 8 provided behind the¼-wavelength plate 7. Only light beams having specific orientations ofpolarization are extracted by polarizing plates 9 a and 9 b, andlight/dark signals are obtained in photosensors 10 a and 10 b.

Since the phases (timings) of the light/dark signals are provided bydeviations in the orientations of polarization of the polarizing plates9 a and 9 b, the phase difference between the light/dark signals is setto 90 degrees by shifting the orientations of polarization of thepolarizing plates 9 a and 9 b by 45 degrees.

A refractive-index-distribution-type lens optical system is used as thereflecting optical element 6, whose length is selected so as to condensean incident parallel light beam at an end surface. A reflecting film iscoated on the end surface.

Such an optical element is called a cat's eye, and has the property ofguiding an incident light beam in the entirely opposite direction. Ingeneral, the above-described encoder has the properties that, when thewavelength of the semiconductor-laser light source 1 changes, thediffraction angle changes to shift the optical path and to change theangle between two light beams to be subjected to interference. As aresult, the state of interference changes.

Furthermore, if the relative alignment between the scale 4 and thedetection head unit shifts, the optical path is also shifted:

However, by using the above-described reflecting optical element 6, thelight beam moves with an original angle even if the diffraction anglechanges, so that the path of the rediffracted light beam does notchange. As a result, stable measurement can be performed.

However, in the above-described conventional approach, the reflectingoptical element 6 must have a size of about 5 mm. Since it is necessaryto project the light beam substantially perpendicularly in order toobtain a predetermined performance, the location to dispose thereflecting optical elements 6 must generally be determined in accordancewith the diffraction angle. In addition, since the reflecting opticalelements 6 are obliquely disposed in the space, reduction in the size ofthe entire encoder is limited.

When the scale 4 is a rotary encoder, a radial diffraction grating isused. In this case, if the light beam is not projected onto a centralportion of the cat's eye, the location irradiated by the returned lightbeam is slightly shifted when the returned light beam is reprojectedonto the diffraction-grating scale 4.

As a result, the phenomenon that the orientation of the rediffractedlight beam is shifted occurs. The influence of this phenomenon is largeras the pitch of the grating is smaller to the order of micrometers andthe diameter of the radial-grating scale (the diameter of the disk) issmaller. In a type in which the scale 4 and the detection unit areseparated, this influence greatly appears as an alignment error due toerrors in the gap between the scale 4 and the detection unit, the angleof installation of these units, and the like. Accordingly, a systemusing a cat's eye has a limitation in the use of a finer radial-gratingscale.

A grating interference encoder of this type adopts a micrometer-orderfine scale. By causing two light beams obtained as a result ofdiffraction by this scale to interfere with each other, a encoder havinga much higher resolution than a geometrical-optics-type encoder isobtained.

This encoder adopts a configuration of generating an interferencepattern by synthesizing the wavefronts of two diffracted light beams.Since the encoder is constituted as an interference optical system, itis necessary to very precisely process respective optical elements andvery precisely dispose these elements Particularly in the case of anassembled encoder in which a scale unit and a detection-head unit areseparated, since the user must mount the scale unit and thedetection-head unit on a motor, a stage or the like, difficulty in themounting operation causes problems.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an optical encoderthat solves the above-described problems and stably detects light beamsusing an optical system for correcting shifts of optical paths, insteadof a cat's eye.

It is another object of the present invention to provide an opticalencoder in which an excellent S/N ratio can be obtained.

According to one aspect, the present invention which achieves theseobjectives relates to a grating interference encoder including alight-emitting device, a diffraction grating for generating twodiffracted light beams having different orders by being irradiated by acoherent light beam from an illuminating optical system, an annulargrating for deflecting the two diffracted light beams having thedifferent orders generated from the diffraction grating to cause thedeflected light beams to be reprojected onto the diffraction grating,and a beam splitter for guiding a light beam, obtained by causingdiffracted light beams generated by rediffraction of the diffractedlight beams reprojected onto the diffraction grating and interfere witheach other, to a photosensor, and the photosensor.

According to another aspect, the present invention which achieves theseobjectives relates to a grating interference encoder including alight-emitting device, a diffraction grating for generating twodiffracted light beams having different orders by being irradiated by acoherent light beam from an illuminating optical system, an annulargrating for deflecting the two diffracted light beams having thedifferent orders generated from the diffraction grating to cause thedeflected light beams to be reprojected onto the diffraction grating, acondenser for condensing the diffracted light beams generated by thediffraction grating onto substantially one point on the annular grating,and a beam splitter for guiding a light beam, obtained by causingdiffracted light beams generated by rediffraction of the diffractedlight beams reprojected onto the diffraction grating to be superposedand interfere with each other, to a photosensor, and the photosensor.

According to still another aspect, the present invention which achievesthese objectives relates to a grating interference encoder including alight-emitting device, a diffraction grating for generating twodiffracted light beams having different orders by being irradiated by acoherent light beam from an illuminating optical system, an annulargrating for deflecting the two diffracted light beams having thedifferent orders generated from the diffraction grating to cause thedeflected light beams to be reprojected onto the diffraction grating, acondenser for making the diffracted light beams generated by thediffraction grating in a state of tending to be condensed onto theannular grating, and a beam splitter for guiding a light beam, obtainedby causing diffracted light beams generated by rediffraction of thediffracted light beams reprojected onto the diffraction grating to besuperposed and interfere with each other, to a photosensor, and thephotosensor.

According to yet another aspect, the present invention which achievesthese objectives relates to a grating-interference-type encoderincluding a light-emitting device, a diffraction grating for generatingtwo diffracted light beams having different orders by being irradiatedby a coherent light beam from an illuminating optical system, an annulargrating for deflecting the two diffracted light beams having thedifferent orders generated from the diffraction grating to cause thedeflected light beams to be reprojected onto the diffraction grating, acondenser for condensing the diffracted light beams generated by thediffraction grating onto substantially one point on the diffractiongrating, and a beam splitter for guiding a light beam, obtained bycausing diffracted light beams generated by rediffraction of thediffracted light beams reprojected onto the diffraction grating to besuperposed and interfere with each other, to a photosensor, and thephotosensor.

According to still a further aspect, the present invention whichachieves these objectives relates to a grating interference encoderincluding a light-emitting device, a diffraction grating for generatingtwo diffracted light beams having different orders by being irradiatedby a coherent light beam from an illuminating optical system, an annulargrating for deflecting the two diffracted light beams having thedifferent orders generated from the diffraction grating to cause thedeflected light beams to be reprojected onto the diffraction grating, acondenser for projecting the diffracted light beams generated by thediffraction grating in a state of tending to be condensed on the annulargrating for causing the diffracted light beams to be diffracted anddeflected, and for condensing the diffracted light beams ontosubstantially one point on the diffraction grating, and a beam splitterfor guiding a light beam, obtained by causing diffracted light beamsgenerated by rediffraction of the diffracted light beams reprojectedonto the diffraction grating to be superposed and interfere with eachother, to a photosensor, and the photosensor.

According to still another aspect, the present invention which achievesthese objectives relates to a grating interference encoder including alight-emitting device, a diffraction grating for generating twodiffracted light beams having different orders by being irradiated by acoherent light beam from an illuminating optical system, an annulargrating for deflecting the two diffracted light beams having thedifferent orders generated from the diffraction grating to cause thedeflected light beams to be reprojected onto the diffraction grating, alinear condenser for linearly condensing the coherent light beam fromthe light-emitting device onto the diffraction grating, and a beamsplitter for guiding a light beam, obtained by causing diffracted lightbeams generated by rediffraction of the diffracted light beamsreprojected onto the diffraction grating to be superposed and interferewith each other, to a photosensor, and the photosensor.

The foregoing and other objects, advantages and features of the presentinvention will become more apparent from the following detaileddescription of the invention taken in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating the configuration of a conventionalencoder;

FIG. 2 is a diagram illustrating the configuration of an encoderaccording to a first embodiment of the present invention;

FIG. 3 is a diagram illustrating the influence of a change in thewavelength of a light beam emitted from a light source;

FIG. 4 is a diagram illustrating a change in the gap between a scale anda detection head;

FIG. 5 is a diagram illustrating the influence of a change in theazimuth angle between the scale and the detection head;

FIGS. 6 and 7 are diagrams, each illustrating the influence of a changein the tilt angle between the scale and the detection head;

FIG. 8 is a diagram illustrating the configuration of an encoderaccording to a second embodiment of the present invention;

FIG. 9 is a diagram illustrating the configuration of an encoderaccording to a third embodiment of the present invention;

FIG. 10 is a diagram illustrating the configuration of an encoderaccording to a fourth embodiment of the present invention;

FIG. 11 is a diagram illustrating the configuration of an encoderaccording to a fifth embodiment of the present invention;

FIGS. 12 and 13 are diagrams, each illustrating a result of tracking ofa light beam;

FIG. 14 is a diagram illustrating the configuration of an encoderaccording to a sixth embodiment of the present invention;

FIGS. 15 and 16 are diagrams, each illustrating a result of tracking ofa light beam;

FIG. 17 is a diagram illustrating the influence of a change in thewavelength of a light beam;

FIG. 18 is a diagram illustrating the influence of a change in the gapbetween a scale and a detection head;

FIGS. 19 and 20 are diagrams, each illustrating the influence of achange in the azimuth angle between the scale and the detection head;

FIGS. 21 through 24 are diagrams, each illustrating the influence of achange in the tilt angle between the scale and the detection head;

FIG. 25 is a diagram illustrating the configuration of an encoderaccording to a seventh embodiment of the present invention;

FIG. 26 is a diagram illustrating the configuration of an encoderaccording to an eighth embodiment of the present invention;

FIG. 27 is a diagram illustrating the configuration of an encoderaccording to a ninth embodiment of the present invention;

FIGS. 28A–28C are diagrams illustrating the configuration of an opticalsystem (condensing of light onto a scale grating by a lens L in a returnpath) according to a tenth embodiment of the present invention;

FIG. 29 is a diagram illustrating the influence of a change in thewavelength of a light beam emitted from a light source in the tenthembodiment;

FIG. 30 is a diagram illustrating a change in the gap between a scaleand a detection head in the tenth embodiment;

FIGS. 31A and 31B are diagrams, each illustrating the influence of achange in the azimuth angle between the scale and the detection head inthe tenth embodiment;

FIGS. 32A–33B are diagrams, each illustrating the influence of a changein the tilt angle between the scale and the detection head in the tenthembodiment;

FIG. 34 is a diagram illustrating addition of an optical system forgenerating a phase-difference signal using a polarizer according to aneleventh embodiment of the present invention;

FIG. 35 is a diagram illustrating an optical path of an encoderaccording to a twelfth embodiment of the present invention (addition ofa convex lens);

FIG. 36 is a diagram illustrating an optical path of an encoderaccording to a thirteenth embodiment of the present invention (additionof a diffraction lens);

FIG. 37 is a perspective view illustrating a fourteenth embodiment ofthe present invention;

FIGS. 38 and 39 are diagrams, each illustrating a result of tracking ofa light beam;

FIG. 40 is a diagram illustrating the influence of a change in thewavelength of a light beam;

FIG. 41 is a diagram illustrating the influence of a change in the gapbetween a scale and a detection head;

FIGS. 42 and 43 are diagrams, each illustrating the influence of achange in the azimuth angle between the scale and the detection head;

FIGS. 44 through 47 are diagrams, each illustrating the influence of achange in the tilt angle between the scale and the detection head;

FIG. 48 is a perspective view illustrating a fifteenth embodiment of thepresent invention;

FIG. 49 is a perspective view illustrating a sixteenth embodiment of thepresent invention; and

FIG. 50 is a perspective view illustrating a seventeenth embodiment ofthe present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will now be described in detail with reference toembodiments shown in FIGS. 2 through 11.

FIG. 2 is a diagram illustrating the configuration of a linear encoderaccording to a first embodiment of the present invention. In FIG. 2, acollimator lens 11, a beam splitter 12, an annular reflection grating14, and a diffraction-grating scale 15 are arranged in the optical pathof a light beam emitted from a semiconductor-laser light source 10, anda photosensor 16 is disposed in a reflecting direction of the beamsplitter 12.

When a grating pitch on the diffraction-grating scale 15 is representedby P1, a pitch P2 of the annular reflection grating 14 is set so as tosatisfy a relationship of P2=P1/2.

According to such a configuration, a light beam L emitted from thesemiconductor-laser light source 10 becomes a substantially parallellight beam by the collimator lens 11, and is projected onto thediffraction-grating scale 15 after passing through central portions ofthe beam splitter 12 and a central portion of the annular reflectiongrating 14. Diffracted light beams L+ and L− reflected from thediffraction-grating scale 15 are projected onto the annular reflectiongrating 14. The annular reflection grating 14 locally operates as alinear diffraction grating having the grating pitch of P2, to projectthe light beams L+ and L− onto the same position on thediffraction-grating scale 15. The light beams L+ and L− are againdiffracted and returned to the beam splitter 12 in a superposed state.

The resultant light beam is guided in a direction different from thesemiconductor-laser light source 10 by the beam splitter 12, and isdetected as an interference light beam by the photosensor 16. When±first-order diffracted light beams are used, the period of light anddark of interference equals four periods with respect to the movement ofone pitch of the diffraction-grating scale 15.

FIG. 3 is a diagram illustrating a shift of the optical path when thediffraction angle changes due to a change in the oscillation wavelengthof the semiconductor-laser light source 10 caused, for example, by achange in the temperature environment. Even if the diffraction angle ofthe diffracted light beam by the diffraction-grating scale 15 changes,since the light beam is diffracted with an original diffraction angle bythe annular reflection grating 14, the position of rediffraction by thediffraction-grating scale 15 and the state of emitted light beams areinvariable. Hence, the state of interference is stable, and no problemsarise even if the grating is replaced by a radial grating.

FIG. 4 is a diagram illustrating a shift of the optical path when theposition of the diffraction-grating scale 15 is shifted. Even if the gapbetween the detection head and the diffraction-grating scale 15 isreduced, since the diffracted light beams from the diffraction-gratingscale 15 are diffracted by the annular reflection grating 14, theposition of rediffraction by the diffraction-grating scale 15 and thestate of emitted light beams are invariable, and the state ofinterference is stable. No problems arise even if the grating isreplaced by a radial grating.

FIG. 5 is a diagram illustrating a shift of the optical path when theangle of installation between the diffraction-grating scale 15 and thedetection-head unit is slightly shifted in an azimuth direction. Even ifan azimuth error is slightly present, the diffracted light beams fromthe diffraction-grating scale 15 are diffracted in the original opticalpaths by the function of the annular diffraction grating 14, theposition of rediffraction by the diffraction-grating scale 15 and thestate of emitted light beams are invariable, and the state ofinterference is stable.

FIG. 6 is a diagram illustrating a shift of the optical path when theangle of installation between the diffraction-grating scale 15 and thedetection-head unit is provided with a tilt error with respect to theline of the grating. Even if a slight amount of tilt error is provided,since the difference between the ±first-order diffracted light beams inthe position of rediffraction and the state of the emitted light beamdoes not change, the state of interference is stable. However, theposition of rediffraction is shifted.

FIG. 7 is a diagram illustrating a shift of the optical path when theangle of installation between the diffraction-grating scale 15 and thedetection-head unit is provided with a tile error with respect to theorientation of arrangement of the grating. Even if a slight amount oftilt error is provided, since the difference between the ±first-orderdiffracted light beams does not change, the state of interference isstable. However, the position of rediffraction is shifted.

As described above, by adopting the annular reflection grating 14 as areflecting optical element, it is possible to obtain a small-size andhigh-resolution encoder having a large allowance for a mounting error.

FIG. 8 is a diagram illustrating the configuration of a secondembodiment of the present invention, in which two-phase signals aredetected by disposing polarizers so as to generate a phase-differencesignal. In FIG. 8, a collimator lens 11, a non-polarizing beam splitter12, an annular reflection grating 14, two polarizing plates 20S and 20Pwhose orientations of polarization are shifted by 90 degrees from eachother, and a diffraction-grating scale 15 are arranged in the opticalpath of a light beam from a semiconductor-laser light source 10, servingas a coherent light source. A ¼-wavelength plate 21 and a non-polarizingbeam splitter 22 are arranged in the reflecting direction of thenon-polarizing beam splitter 12. A polarizing plate 23 a and aphotosensor 16 a are disposed in the reflecting direction of thenon-polarizing beam splitter 22, and a polarizing plate 23 b and aphotosensor 16 b are arranged in the transmitting direction of thenon-polarizing beam splitter 22.

According to the above-described configuration, a light beam L from thesemiconductor-laser light source 10 passes through the collimator lens11 and the non-polarizing beam splitter 12, and is substantiallyperpendicularly projected onto the diffraction-grating scale 15 afterpassing through a central transmitting window portion of the annularreflection grating 14. A +first-order diffracted light beam reflectedfrom the diffraction-grating scale 15 is emitted with a diffractionangle θ, is diffracted and reflected to the original optical path by theannular reflection grating 14, and is returned to the non-polarizingbeam splitter 12 by being subjected to +first-order diffraction by thediffraction-grating scale 15.

A −first-order diffracted light beam reflected from thediffraction-grating scale 15 is diffracted and reflected to the originaloptical path by the annular reflection grating 14, and is returned tothe non-polarizing beam splitter 12 by being subjected to −first-orderdiffraction by the diffraction-grating scale 15. The light beamprojected from the semiconductor-laser light source 10 onto thediffraction-grating scale 15 has vertically and horizontally polarizedcomponents. Although the ±first-order diffracted light beams propagatedto the non-polarizing beam splitter 12 are not light/dark light beams,the orientations of polarization of these light beams are shifted by 90degrees from each other and the wavefronts of these light beams aresuperposed.

As a result, the two light beams reflected by the non-polarizing beamsplitter 12 pass through the ¼-wavelength plate 21 and is converted intoa linearly polarized light beam whose orientation of polarizationchanges based on the phase difference between the two light beams. Theobtained light beam is divided into two light beams by thenon-polarizing beam splitter 22 provided behind the ¼-wavelength plate21. Only specific orientations of polarization are extracted by thepolarizing plates 23 a and 23 b, and light/dark signals are sensed bythe photodetectors 16 a and 16 b. The phases (timings) of theselight/dark signals are provided by shifts of the orientation ofpolarization of the polarizing plates 23 a and 23 b. That is, byshifting the orientations of polarization of the polarizing plates 23 aand 23 b by 45 degrees in opposite directions, the phase differencebetween the light-dark signals is set to 90 degrees.

FIG. 9 is a diagram illustrating the configuration of a third embodimentof the present invention, in which two-phase signals are detected bydisposing a crystal optical element so as to generate a phase-differencesignal. In the third embodiment, a ¼-wavelength plate 24 is inserted inone of optical paths between a diffraction-grating scale 15 and anannular reflection grating 14.

A light beam from a semiconductor-laser light source 10 is substantiallyperpendicularly projected onto the diffraction-grating scale 15 afterpassing through a non-polarizing beam splitter 12. A +first-orderdiffracted light beam reflected from the diffraction-grating scale 15 isemitted with a diffraction angle θ, reaches an annular reflectiongrating 14 after passing through the ¼-wavelength plate 24, isdiffracted and reflected to the original optical path by the annularreflection grating 14, and is returned to the non-polarizing beamsplitter 12 by being subjected to +first-order diffraction by thediffraction-grating scale 15.

A −first-order diffracted light beam reflected from thediffraction-grating scale 15 is emitted with a diffraction angle θreaches the annular reflection grating 14 after passing through the¼-wavelength plate 24, is diffracted and reflected to the originaloptical path by the annular reflection grating 14, and is returned tothe non-polarizing beam splitter 12 by being subjected to −first-orderdiffraction by the diffraction-grating scale 15. The polarized componentof the light beam projected from the semiconductor-laser light source 10onto the diffraction-grating scale 15 makes an angle of 45 degrees withrespect to the optical axis of the ¼-wavelength plate 24, and theorientation of polarization is shifted by 90 degrees only for a lightbeam passing back and forth through the ¼-wavelength plate 24. Hence,although the ±first-order diffracted light beams propagated to thenon-polarizing beam splitter 12 are not light/dark light beams, thewavefronts of these light beams are superposed in a state in which theorientations of polarization are shifted by 90 degrees from each other.

Then, as in the second embodiment, these light beams pass through a¼-wavelength plates 21 and a non-polarizing beam splitter 22, andlight/dark signals are sensed by photosensors 16 a and 16 b viapolarizing plates 23 a and 23 b, respectively.

FIG. 10 is a diagram illustrating the configuration of a fourthembodiment of the present invention, in which two-phase signals aredetected by disposing a crystal optical element so as to generate aphase-difference signal. In the fourth embodiment, ⅛-wavelength plates25 a and 25 b are inserted in optical paths between adiffraction-grating scale 15 and an annular reflection grating 14 in astate in which the optical axes are shifted by 90 degrees from eachother.

A light beam L from a semiconductor-laser light source 10 issubstantially perpendicularly projected onto the diffraction-gratingscale 15 after passing through a non-polarizing beam splitter 12. A+first-order diffracted light beam reflected from thediffraction-grating scale 15 is emitted with a diffraction angle θ,reaches the annular reflection grating 14 after passing through the⅛-wavelength plate 25 a, is diffracted and reflected to the originaloptical path by the annular reflection grating 14, and is returned tothe non-polarizing beam splitter 12 by being subjected to +first-orderdiffraction by the diffraction-grating scale 15.

A −first-order diffracted light beam reflected from thediffraction-grating scale 15 is emitted with a diffraction angle θ,reaches the annular reflection grating 14 after passing through the⅛-wavelength plate 25 b, is diffracted and reflected to the originaloptical path by the annular reflection grating 14, and is returned tothe non-polarizing beam splitter 12 by being subjected to −first-orderdiffraction by the diffraction-grating scale 15. The polarized componentof the light beam projected from the semiconductor-laser light source 10onto the diffraction-grating scale 15 makes an angle of 45 degrees withrespect to the optical axis of the ⅛-wavelength plates 25 a and 25 b.

The ±first-order diffracted light beams propagated to the non-polarizingbeam splitter 12 are circularly polarized light beams which circulate inopposite directions Hence, when these light beams are subjected tovector synthesis, they are converted into a linearly polarized lightbeam in which the orientation of polarization changes based on the phasedifference between these light beams. The obtained light beam is dividedinto two light beams by a non-polarizing beam splitter 22 providedbehind the non-polarizing beam splitter 12. Only specific orientationsof polarization are extracted by polarizing-plates 23 a and 23 b, andlight/dark signals are sensed by photosensors 16 a and 16 b.

FIG. 11 is a diagram illustrating the configuration of a fifthembodiment of the present invention. In FIG. 11, an annular transmissiongrating 214 is adopted instead of the annular reflection grating 14. Alight beam is reflected by a reflecting surface 215 provided immediatelybehind the annular transmission grating 214, and is reprojected onto adiffraction-grating scale 15 after being subjected to diffraction twice.The pitch of the annular transmission grating 214 is set to the samevalue as the pitch of the diffraction-grating scale 15.

As in the fourth embodiment, a light beam from a semiconductor-laserlight source 10 is projected onto the diffraction-grating scale 15 afterpassing through a non-polarizing beam splitter 12. A +first-orderdiffracted light beam reflected from the diffraction-grating scale 15 isemitted with a diffraction angle θ, is diffracted by the annulartransmission grating 214 after passing through a ⅛-wavelength plate 25a, is returned to the original optical path by the reflecting surface215 immediately after diffraction, is again diffracted and deflected bythe annular transmission grating 214, and is returned to thenon-polarizing beam splitter 12 by being subjected to +first-orderdiffraction by the diffraction-grating scale 15.

A −first-order diffracted light beam reflected from thediffraction-grating scale 15 is emitted with a diffraction angle θ, isdiffracted by the annular transmission grating 214 after passing througha ⅛-wavelength plate 25 b, is returned to the original optical path bythe reflecting surface 215, is again diffracted and deflected by theannular transmission grating 214, and is returned to the non-polarizingbeam splitter 12 by being subjected to −first-order diffraction by thediffraction-grating scale 15.

As in the fourth embodiment, the ±first-order diffracted light beams areconverted into a linearly polarized light beam, which is divided intotwo light beams by a non-polarizing beam splitter 22, and light/darksignals are sensed by photosensors 16 a and 16 b via the polarizingplates 23 a and 23 b, respectively.

In FIG. 11, in order to obtain a phase-difference signal, an approach ofinserting polarizing plates as shown FIG. 8, or an approach of insertinga phase-difference plate as shown in FIG. 9 may also be adopted. Thesepolarizing-state changing elements may be inserted between the annulartransmission grating 214 and the reflecting surface 215.

In the above-described embodiments, partial modification may beperformed with respect to the following items.

(a) In the diffraction-grating scale 15 and the annular reflectiongrating 14, diffracted light beams having a diffraction order other thanthe ±first-order diffracted light beams are used.

(b) The, polarizing plates 20S, 20P, 23 a and 23 b are replaced byprisms, each having a polarizing film, or fine-grating patterns, servingas other elements having the equivalent functions.

(c) The phase-difference plates, i.e., the ¼-wavelength plate 21 and the⅛-wavelength plates 25 a and 25 b are replaced by fine-structurepatterns or other anisotropic materials having functions equivalent tothe functions of a crystal optical element, such as quartz or the like.

(d) The same effects are obtained by providing at least two phases for aphase-difference signal and setting the phase difference to a valueother than 90 degrees, and partially changing the arrangement ofpolarizers or phase-difference plates.

(e) In the above-described embodiments, the non-polarizing beamsplitters 12 and 22 are used in order to guide light beams to beprojected onto the diffraction-grating scale 15 and to guiderediffracted light beams to the photosensor 16, respectively. However,the light beams may be guided by using any other appropriate beamdividing/synthesizing means, such as diffraction gratings or the like,or by separating the light beams by spatially shifting forward andbackward optical paths, or by selectively reflecting only one of thelight beams and guiding the selected light beam to the photosensor 16.

(f) By replacing the diffraction-grating scale 15 by a disc-shaped scalehaving a radial grating, the above-described encoder can be easilychanged to a rotary encoder.

In the above-described embodiments, for example, an element having areflecting film deposited in the vacuum on the back of a glass plateprocessed by etching or the like can be used as the annular reflectiongrating. Hence, an excellent environment resisting property is obtained.

As described above, the optical encoder according to the presentinvention has the following effects by optimizing a state of projectionof a light beam onto a diffraction-grating scale or an annulardiffraction grating.

(1) Since, for example, a plane optical element can be used, the spaceof arrangement is not complicated as in the case of using a cat's eye,and a small encoder can be easily provided.

(2) Since variations in the wavelength of the light beam from the lightsource are corrected, an interference signal is stabilized.

(3) Since an alignment error is corrected, even an encoder in which adiffraction-grating scale and a detection head are separated can berelatively easily mounted.

(4) The size of a beam reflecting optical element is small and thenumber of components is small. Hence, by adding deflection means tolight-beam projection means, the degree of freedom in the method or thedirection of projection of a light beam onto a diffraction-grating scaleis increased, and a wider posture of application can be obtained.

In the above-described embodiments, since the light beam actuallyprojected onto the diffraction-grating scale 5 has a finite size, iftracking of a light beam when, for example, using thediffraction-grating scale 5 having a pitch of 2.8 μm, and setting thedistance between the diffraction-grating scale 5 and the annularreflection grating 4 to 10 mm is performed, an elliptic wavefront isobtained as shown in FIG. 12, due to a wavefront distortion peculiar tothe annular reflection grating 4, and a loss may be produced when thephotosensor 6 senses the light beam.

Furthermore, if tracking of a light beam having a diameter of 1 mm whenprojecting the light beam onto a radial grating having a radius of 9.2mm and 20,250 grooves at the circumference is performed, then, as shownin FIG. 13, a wavefront distortion peculiar to a radial grating is addedto the wavefront distortion peculiar to the annular reflection grating4, and the wavefronts of ±first-order diffracted light beams may bedistorted when synthesizing these light beams.

The following embodiments have been made in consideration of theabove-described problems.

A description will now be provided in detail for the embodiments shownin FIGS. 14 through 27.

FIG. 14 is a diagram illustrating the configuration of a linear encoderaccording to a sixth embodiment of the present invention. In FIG. 14, acollimator lens 11, a beam splitter 12, a lens 13, an annular reflectiongrating 14, and a diffraction-grating scale 115 are arranged in theoptical path of a light beam emitted from a semiconductor-laser lightsource 10, and a photosensor 16 is disposed in the reflecting directionof the beam splitter 12.

When the pitch of the grating on the diffraction-grating, scale 115 isrepresented by P1, the pitch P2 of the annular reflection grating 14 isset so as to satisfy a relationship of P2=P1/2.

According to such a configuration, a light beam L emitted from thesemiconductor-laser light source 10 becomes a substantially parallellight beam by the collimator lens 11, and is condensed and projectedonto the diffraction-grating scale 115 after passing through the beamsplitter 12, the lens 13 and a central portion of the annular reflectiongrating 14. Diffracted light beams L+ and L−reflected from thediffraction-grating scale 115 are projected onto substantially one pointon the annular reflection grating 14. When using a radialdiffraction-grating scale as the diffraction-grating scale 115 as shownin FIG. 14, the light beams are not completely condensed on one pointdue to an aberration peculiar to a radial grating.

Even when using a linear diffraction-grating scale, the light beamcannot be condensed on an area less than the beam waste size of thelaser beam. However, since this size is very small, it can be consideredas one point.

The annular reflection grating 14 locally operates as a lineardiffraction grating having the pitch of P2. Hence, the light beamsemitted from the condensed and projected region of thediffraction-grating scale 115 and projected onto substantially one pointon the annular reflection grating 14 are diffracted and returned to theoriginal optical path to be projected onto and diffracted by thediffraction-grating scale 115, and are returned to the beam splitter 12in a superposed state. The resultant light beam is guided in a directiondifferent from the semiconductor-laser light source 10 by the beamsplitter 12, and is detected as an interference light beam by thephotosensor 16. When ±first-order diffracted light beams are used, theperiod of light and dark of interference equals four periods withrespect to one pitch of the diffraction-grating scale 115. As shown inFIG. 15 or 16, a substantially circular light beam is obtained on thephotosensor 16.

FIG. 17 is a diagram illustrating a result of calculating a shift of theoptical path when the diffraction angle changes due to a change in theoscillation wavelength of the semiconductor-laser light source 10 by

λ=10 nm caused, for example, by a change in the temperature environment.

In this case, the irradiated position on the annular reflection grating14 is slightly shifted due to a change in the diffraction angle of thediffracted light beam by the diffraction-grating scale 115. However,since the light beam is diffracted with an original diffraction angle bythe annular reflection grating 14, the position of rediffraction by thediffraction-grating scale 15 and the state of the emitted light beam areinvariable. Hence, the state of interference is stable.

FIG. 18 is a diagram illustrating a shift of the optical path when theposition of the radial-grating disk of the diffraction-grating scale 115is shifted by

x=0.5 mm. Even if the gap between the detection-head unit and thediffraction-grating scale 115 is reduced, the position of rediffractionby the diffraction-grating scale 115 and the state of the emitted lightbeam are invariable. Hence, the state of interference is stable.

FIGS. 19 and 20 are diagrams, each illustrating a result of calculationwhen the detection-head unit is shifted with respect to theradial-grating disk by

y=0.5 mm in a tangential direction. This case is equivalent to the casethat the radial-grating disk is eccentric by 0.5 mm, and is alsoequivalent to an azimuth error in the arrangement of the scale whenusing a linear grating. Even if the irradiated position is shifted,since the light beam is diffracted to original optical path by thefunction of the annular reflection grating 14, the position ofrediffraction by the diffraction-grating scale 15 and the state of theemitted light beam are invariable.

FIGS. 19 and 20 illustrates the optical paths of a +first-orderdiffracted light beam and a −first-order diffracted light beam,respectively. Although the irradiated positions on the photosensor 16are slightly shifted in the vertical direction, these light beams aresubstantially parallel to each other, and the state of interference isstable. The above-described amount of shift of 0.5 mm is provided inorder to facilitate understanding of the result of calculation. Theamount of shift in the actual encoder is much smaller.

FIGS. 21 and 22 are diagrams, each illustrating a shift of the opticalpath when a tilt error of

θz=0.5 degrees is given with respect the orientation of arrangement ofthe grating. The result of reading of the optical paths of the±first-order diffracted light beams indicates that, even if a smallamount of tilt error is added, the difference between the ±first-orderdiffracted light beams in the position of rediffraction by thediffraction-grating scale 15 and the state of emitted light beams doesnot change. Hence, the state of interference is stable.

In FIGS. 21 and 22, the incident light beams on the photosensor 16 areshifted from the surface of the photosensor 16. However, the amount ofshift of 0.5 mm is provided in order to facilitate understanding of theresult of calculation. The amount of shift in the actual encoder is muchsmaller.

FIGS. 23 and 24 are diagrams, each illustrating a shift of the opticalpath when a tilt error of

θy=0.5 degrees is given with respect the orientation of arrangement ofthe grating. Also in this case, the result of reading of the opticalpaths of the ±first-order diffracted light beams indicates that, even ifa small amount of tilt error is added, the difference between the±first-order diffracted light beams in the position of rediffraction bythe diffraction-grating scale 15 and the state of emitted light beamsdoes not change. Hence, the state of interference is stable, and theirradiated position on the photosensor 16 is substantially not shifted.

As described above, by combining the annular reflection diffractiongrating 14 with projection of a light beam on a point of this opticalelement, it is possible to make provision of a small-size andhigh-resolution encoder having a large allowance in a mounting error tobe compatible with detection of a stable interference signal.

FIG. 25 is a perspective view illustrating a seventh embodiment of thepresent invention, in which two-phase signals are detected by disposinga polarizing element so as to generate a phase-difference signal.Although a linear encoder using a polarizing plate as a polarizingelement, and using a linear diffraction lens is illustrated, a rotaryencoder using a ¼-wavelength plate as a polarizing element and using aradial diffraction lens may also be adopted.

In FIG. 25, a collimator lens 11, a non-polarizing beam splitter 12, alens 13, an annular reflection grating 14, two polarizing plates 20S and20P whose orientations of polarization are shifted by 90 degrees fromeach other, and a diffraction-grating scale 15 are arranged in theoptical path of a light beam from a semiconductor-laser light source 10,serving as an coherent light source. A ¼-wavelength plate 21 and anon-polarizing beam splitter 22 are arranged in the reflecting directionof the non-polarizing beam splitter 12. A polarizing plate 23 a and aphotosensor 16 a are disposed in the reflecting direction of thenon-polarizing beam splitter 22, and a polarizing plate 23 b and aphotosensor 16 b are arranged in the transmitting direction of thenon-polarizing beam splitter 22.

According to the above-described configuration, a light beam from thesemiconductor-laser light source 10 passes through the collimator lens11 and the non-polarizing beam splitter 12, and is substantiallyperpendicularly projected onto the diffraction-grating scale 15 afterpassing through the lens 13 and a central portion of the annularreflection grating 14.

A +first-order diffracted light beam reflected from thediffraction-grating scale 15 is emitted with a diffraction angle θ, isdiffracted and reflected to the original optical path by the annularreflection grating 14, and is returned to the non-polarizing beamsplitter 12 by being subjected to +first-order diffraction by thediffraction-grating scale 15.

A −first-order diffracted light beam reflected from thediffraction-grating scale 15 is emitted with a diffraction angle θ, isdiffracted and reflected to the original optical path by the annularreflection grating 14, and is returned to the non-polarizing beamsplitter 12 by being subjected to −first-order diffraction by thediffraction-grating scale 15. The light beam projected from thesemiconductor-laser light source 10 onto the diffraction-grating scale15 has vertically and horizontally polarizing components, and theorientations of polarization of the ±first-order diffracted light beamspropagated to the non-polarizing beam splitter 12 are shifted by 90degrees from each other and the wavefronts of these light beams aresuperposed. However, these light beams are not light/dark light beams.

As a result, the two light beams reflected by the non-polarizing beamsplitter 12 pass through the ¼-wavelength plate 21 and are convertedinto a linearly polarized light beam whose orientation of polarizationchanges based on the phase difference between the two light beams. Theobtained light beam is divided into two light beams by thenon-polarizing beam splitter 22 provided behind the ¼-wavelength plate21. Only specific orientations of polarization are extracted by thepolarizing plates 23 a and 23 b, and light/dark signals are sensed bythe photodetectors 16 a and 16 b. The phases (timings) of theselight/dark signals are provided by shifts of the orientation ofpolarization of the polarizing plates 23 a and 23 b. That is, byshifting the orientations of polarization of the polarizing plates 23 aand 23 b by 45 degrees in opposite directions, the phase differencebetween the light-dark signals is set to 90 degrees.

FIG. 26 is a perspective view illustrating an eighth embodiment of thepresent invention, in which a convergent light beam is directly obtainedby using a collimator lens 11 as optical means for projecting aconvergent light beam. In the case of FIG. 26, a very graduallyconvergent light beam is obtained.

FIG. 27 is a diagram illustrating the configuration of a ninthembodiment of the present invention, in which a diffraction lens 25 isintegrally formed at a central portion (a transmitting-window portion)of an annular reflection grating 14. The diffraction lens 25 ispatterned so that the pitch of the grating gradually changes from thecentral portion to the peripheral portion, in order to provide thefunction of a convex lens.

In the above-described embodiments, partial modification may beperformed with respect to the following items.

(a) In the diffraction-grating scale 15 and the radial diffractiongrating 115 or the annular reflection grating 14, diffracted light beamshaving a diffraction order other than the ±first-order diffracted lightbeams are used.

(b) The polarizing plates 20S, 20P, 23 a and 23 b are replaced byprisms, each having a polarizing film, or fine-grating patterns, servingas other elements having the equivalent functions.

(c) The phase-difference plates, i.e., the ¼-wavelength plate 21 and the⅛-wavelength plates are replaced by fine-structure patterns or otheranisotropic materials having functions equivalent to the functions of acrystal optical element, such as quartz or the like.

(d) The same effects are obtained by providing at least two phases for aphase-difference signal and setting the phase difference to a valueother than 90 degrees, and partially changing the arrangement ofpolarizers or phase-difference plates.

(e) In the above-described embodiments, the non-polarizing beamsplitters 12 and 22 are used in order to guide light beams to beprojected onto the diffraction-grating scale 15 and to guiderediffracted light beams to the photosensor 16, respectively. However,the light beams may be guided by using any other appropriate beamdividing/synthesizing means, such as diffraction gratings or the like,or by separating light beams by spatially shifting forward and backwardoptical paths, or by selectively reflecting only one of the light beamsand guiding the selected light beam to the photosensor 16.

(f) For example, by changing the order of arrangement of the collimatorlens 11, the non-polarizing beam splitter 12, the lens 13 and theannular reflection grating 14, another optical arrangement is adopted inorder to provide a system of linearly condensing a light beam onto thediffraction-grating scale 15.

In the above-described embodiments, for example, an element having areflecting film deposited in the vacuum on the back of a glass plateprocessed by etching or the like, can be used as the annular reflectiongrating 14. Hence, an excellent environment resisting property isobtained.

As described above, the optical encoder according to the presentinvention has the following effects by optimizing a state of projectionof a light beam onto a diffraction-grating scale or an annulardiffraction grating.

(1) The interfered wavefronts of the diffracted light beams tend tocoincide with each other, a flat light-dark pattern is obtained, and astable encoder signal having an excellent S/N ratio can be obtained.

(2) Since, for example, a plane optical element can be used as theannular reflection grating, the space of arrangement is not complicated,and a small encoder can be easily provided.

(3) Since variations in the wavelength of the light beam from the lightsource are corrected, an interference signal is stabilized.

(4) Since an alignment error is corrected, even an encoder in which adiffraction-grating scale and a detection head are separated can berelatively easily mounted.

(5) The size of a beam reflecting optical element is small and thenumber of components is small. Hence, by adding deflection means tolight-beam projection means, the degree of freedom in the method or thedirection of projection of a light beam onto a diffraction-grating scaleis increased, and a wider posture of application can be obtained.

(6) Since the rediffracted light beam is guided to the photosensorwithout greatly spreading, it is possible to perform detection with asmall loss and an excellent S/N ratio.

FIG. 28A is a diagram illustrating the arrangement of the optical systemof a rotary encoder according to a tenth embodiment of the presentinvention. FIGS. 28B and 28C are diagrams illustrating results oftracking of + or −first-order light beam by projecting each light beamhaving a diameter of 1 mm onto the same optical system. A light beam Remitted from a semiconductor laser becomes a substantially parallellight beam by a lens 13, passes through a beam splitter 12, becomes aconverged light beam after passing through the lens 13, and is projectedonto a radial diffraction-grating scale 115 in a state of tending to beconverged. Diffracted light beams R+ and R− from the diffraction-gratingscale 115 are projected onto an annular reflection grating 14. When thepitch of the grating on the diffraction-grating scale 115 is representedby P1, the pitch P2 of the annular reflection grating 14 is set to beP2=P1/2.

The annular reflection grating 14 locally operates as a lineardiffraction grating having the pitch P2. Hence, the light beam emittedfrom a linear condensing region of the radial diffraction-grating scale115 and projected onto the annular reflection grating 14 is diffractedsubstantially in the original direction toward the radialdiffraction-grating scale 115. At that time, the light beam is condensedon substantially one point by the function of the lens 13, and isrediffracted in a state of tending to be diverged. The diffracted lightbeams are superposed and returned to the beam splitter 12.

As for the degree of point-like condensation on the diffraction-gratingscale in the return path, there exists a linear condensation aberrationby the annular reflection grating 14, and when using the radialdiffraction-grating scale 115 as the diffraction-grating scale, anaberration peculiar to a radial grating is also added. Accordingly,completely point-like condensation cannot be realized. However, such asmall amount of aberration can be neglected.

In the tenth embodiment, it is designed to realize point-likecondensation on the diffraction-grating scale in the return path whenthe radial diffraction-grating scale 115 and the annular reflectiongrating 14 locally operate as liner gratings. Actually, however,complete point-like condensation is not realized due to theabove-described aberrations. The position of condensation whenneglecting the above-described aberrations need not completely coincidewith the initial position on the diffraction-grating scale in the returnpath. The effect of stabilizing the state of interference is obtainedprovided that the position of condensation is between the annularreflection grating 14 and the radial diffraction-grating scale 115.

The resultant light beam is guided in a direction different from thesemiconductor laser 10 by the beam splitter 12, and is detected as aninterference light beam by a photosensor 16. When using ±first-orderdiffracted light beams, the period of light and dark of interferenceequals four periods for the movement of 1 pitch of the radialdiffraction-grating scale 115.

FIG. 28B illustrates a result of calculation for light-beam elementsonly in the direction of arrangement of the grating at irradiatedportions on the radial diffraction-grating scale 115 and the annularreflection grating 14. In FIG. 28B, only light-beam elements between theannular reflection grating 14 and the photosensor 16 are selectivelyillustrated. As shown in FIG. 28B, the light beam is substantiallycondensed at the radial diffraction-grating scale 115 in the returnpath, and is directed from the radial diffraction-grating scale 115 tothe photosensor 16 as a divergent light beam. FIG. 28C illustrates aresult of tracking of parallel light-beam elements having a diameter of1 mm from a light source 10 to the photosensor 16. A substantiallyelliptical light beam is obtained on the photosensor 16.

FIG. 29 is a diagram illustrating a result of calculating a shift of theoptical path when the diffraction angle changes due to a change in theoscillation wavelength of the semiconductor-laser light source 10 by 10nm caused, for example, by a change in the temperature environment, inthe tenth embodiment. In this case, the irradiated position on theannular reflection grating 14 is slightly shifted due to the change inthe diffraction angle. However, since the light beam is diffracted withan original diffraction angle by the annular reflection grating 14, theposition of rediffraction by the radial diffraction-grating scale 115and the state of the emitted light beam are invariable. Hence, the stateof interference is, of course, stable.

FIG. 30 is a diagram illustrating a shift of the optical path when theposition of the radial diffraction-grating scale 115 is shifted by 0.5mm. Even if the gap between the detection-head unit and the radialdiffraction-grating scale 115 is increased, the position ofrediffraction of the light beams diffracted by the annular reflectiongrating 14 by the radial diffraction-grating scale 115 and the state ofthe emitted light beam are invariable. Hence, the state of interferenceis, of course, stable.

FIGS. 31A and 31B are diagrams, each illustrating a result ofcalculation when the detection-head unit is shifted with respect to theradial diffraction-grating scale 115 by 0.5 mm in a tangentialdirection, in the tenth embodiment. This case is equivalent to the casethat the radial diffraction-grating scale 115 is eccentric by 0.5 mm,and is also equivalent to an azimuth error in the arrangement of thescale when using the linear grating 15. Even if the irradiated positionis shifted, since the light beam is diffracted to the original opticalpath by the annular reflection grating 14, the position of rediffractionby the radial diffraction-grating scale 115 and the state of the emittedlight beam are invariable. FIGS. 31A and 31B illustrate the opticalpaths of a +first-order diffracted light beam and a −first-orderdiffracted light beam, respectively. Although the irradiated positionson the photosensor 16 are slightly shifted in the vertical direction,these light beams are substantially parallel to each other, and thestate of interference is stable. The above-described amount of shift of0.5 mm is provided in order to facilitate understanding of the result ofcalculation. The amount of shift in the actual encoder is much smaller.

FIGS. 32A and 32B are diagrams, each illustrating a shift of the opticalpath when a tilt error of

θz=0.5 degrees is given with respect the line of the grating for theangle of installation between the radial diffraction-grating scale 115and the detection-head unit. The result of reading of the optical pathsof the ±first-order diffracted light beams shown in FIGS. 32A and 32Bindicates that, even if a small amount of tilt error is added, thedifference between the ±first-order diffracted light beams in theposition of rediffraction by the radial diffraction-grating scale 115does not change. Hence, the state of interference is stable. In FIGS.32A and 32B, the incident light beams on the photosensor 16 are shiftedin reverse directions in the z-axis direction on the surface of thephotosensor 16. Since these z-axis components are substantiallyparallel, the state of interference hardly changes. The amount of shiftof 0.5 mm in the calculation is provided in order to facilitateunderstanding of the result of calculation. The amount of shift in theactual encoder is much smaller.

FIGS. 33A and 33B are diagrams, each illustrating a shift of the opticalpath when a tilt error of

θy=0.5 degrees is provided with respect the orientation of arrangementof the grating for the angle of installation between the radialdiffraction-grating scale 115 and the detection-head unit. The result ofreading of the optical paths of the ±first-order diffracted light beamsshown in FIGS. 33A and 33B indicates that, even if a small amount oftilt error is added, the difference between the ±first-order diffractedlight beams in the position of rediffraction by the radialdiffraction-grating scale 115 does not change. Hence, the state ofinterference is stable, and the irradiated position on the photosensor16 is substantially not shifted.

As described above, by combining the annular reflection grating 14 withpoint-like projection of a light beam on the radial diffraction-gratingscale 115 in the return path, it is possible to make provision of asmall-size and high-resolution encoder having a large allowance in amounting error to be compatible with detection of a stable interferencesignal.

FIG. 34 is a diagram illustrating an eleventh embodiment of the presentinvention, in which two-phase signals are detected by disposing apolarizing element so as to generate a phase-difference signal in theconfiguration of the tenth embodiment. In the eleventh embodiment shownin FIG. 34, a linear encoder using a polarizing plate as the polarizingelement and using a linear diffraction lens is adopted. In FIG. 34, alight beam from a semiconductor laser 10 passes through a non-polarizingbeam splitter 12, and is substantially perpendicularly projected onto adiffraction-grating scale 15.

A +first-order diffracted light beam reflected from thediffraction-grating scale 15 is emitted with a diffraction angle θ, isdiffracted and reflected to the original optical path by an annularreflection grating 14 provided at an upper portion, and is returned tothe non-polarizing beam splitter 12 by being subjected to +first-orderdiffraction by the diffraction-grating scale 15.

A −first-order diffracted light beam reflected from thediffraction-grating scale 15 is emitted in the opposite direction with adiffraction angle θ, is diffracted and reflected to the original opticalpath by the annular reflection grating 14, and is returned to thenon-polarizing beam splitter 12 by being subjected to −first-orderdiffraction by the diffraction-grating scale 15. In the eleventhembodiment shown in FIG. 34, polarizing plates 20 s and 20 p whoseorientations of polarization are shifted by 90 degrees from each otherare inserted in -the optical path between the diffraction-grating scale15 and the annular reflection grating 14. The light beam projected fromthe semiconductor laser 10 onto the diffraction-grating scale 115includes the above-described polarized components.

Accordingly, the orientations of polarization of the ±first-orderdiffracted light beams propagated to the non-polarizing beam splitter 12are shifted by 90 degrees from each other and the wavefronts of theselight beams are superposed (not light/dark light beams).

As a result, the two light beams reflected by the non-polarizing beamsplitter 12 pass through a ¼-wavelength plate 21 and are converted intoa linearly polarized light beam whose orientation of polarizationchanges based on the phase difference between the two light beams. Theobtained light beam is divided into two light beams by a non-polarizingbeam splitter 22 provided behind the ¼-wavelength plate 21. Byextracting only specific orientations of polarization by polarizingplates 23 a and 23 b, light/dark signals are obtained. The phases(timings) of these light/dark signals are provided by shifts of theorientation of polarization of the polarizing plates 23 a and 23 b. Byshifting the orientations of polarization of the polarizing plates 23 aand 23 b by 45 degrees in opposite directions, the phase differencebetween the light-dark signals is set to 90 degrees.

FIG. 35 illustrates a twelfth embodiment of the present invention. InFIG. 35, instead of adopting a lens 13 as optical means for projecting aconvergent light beam, a convergent light beam is directly obtained by acollimator lens 11 (not shown). In FIG. 36 illustrating a thirteenthembodiment of the present invention, a diffraction lens is integrallyformed at a central portion (transmitting-window portion) of an annularreflection grating 14. The diffraction lens is patterned so that thepitch of the grating gradually changes from the central portion to theperipheral portion, in order to provide the function of a convex lens.

In the tenth, eleventh and twelfth embodiments, partial modification maybe performed with respect to the following items.

(1) In the diffraction-grating scale 15 or the radial diffractiongrating 115 and the annular reflection grating 14, diffracted lightbeams having a diffraction order other than the ±first-order diffractedlight beams are used.

(b) The polarizing plates are replaced by other elements (prisms, eachhaving a polarizing film, or fine-grating patterns) having theequivalent functions.

(3) The phase-difference plates (the ¼-wavelength plate and the⅛-wavelength plates) are replaced by elements (fine-structure patternsor other anisotropic materials) having functions equivalent to thefunctions of a crystal optical element, such as quartz or the like.

(4) The same effects are obtained by providing at least two phases for aphase-difference signal and setting the phase difference to a valueother than 90 degrees, and partially changing the arrangement ofpolarizing elements or phase-difference plates.

(5) In the above-described embodiments, the non-polarizing beamsplitters are used in order to guide light beams to be projected ontothe diffraction-grating scale and to guide rediffracted light beams tothe photosensor. However, the light beams may be guided by using anyother appropriate beam dividing/synthesizing means (diffraction gratingsor the like), or by separating light beams by spatially shifting forwardand backward optical paths, or by selectively reflecting only one of thelight beams and guiding the selected light beam to the photosensor.

(6) For example, by changing the order of arrangement of the collimatorlens, the non-polarizing beam splitter and the annular reflectiongrating, another optical arrangement is adopted in order to provide asystem of condensing a light beam onto one point of thediffraction-grating scale in the return path.

The tenth, eleventh and twelfth embodiments have the following effectsby using a convergent-light-beam projection optical system, and anannular reflection grating as a returning optical element.

(1) Even when using a radial diffraction grating as a scale grating, theinterfered wavefronts of the diffracted light beams tend to coincidewith each other, a flat light-dark pattern is obtained, and a stableencoder signal having an excellent S/N ratio can be obtained.

(2) Since a plane optical element is used as the annular reflectiongrating, the space of arrangement is not complicated, and a smallencoder can be easily provided.

(3) Since variations in the wavelength of the light beam from the lightsource are corrected, an interference signal is stabilized.

(4) Since an alignment error is corrected, even an encoder in which ascale grating and a detection head are separated can be relativelyeasily mounted.

(5) Since an element having a reflecting film deposited in the vacuum onthe back of a glass plate processed by etching or the like can be usedas the annular reflection grating, an excellent environment resistingproperty is provided.

(6) The size of a beam reflecting optical element is small and thenumber of components is small. Hence, by adding deflection means tolight-beam projection means, the degree of freedom in the method or thedirection of projection of a light beam onto a scale grating isincreased, and a wider posture of application can be obtained.

(7) Since the rediffracted light beam is guided to the photosensorwithout spreading, it is possible to perform detection with a small lossand an excellent S/N ratio.

Fourteenth through seventeenth embodiments of the present invention willnow be described in detail with reference to FIGS. 37 through 50.

FIG. 37 is a perspective view illustrating the configuration of a rotaryencoder according to a fourteenth embodiment of the present invention.In FIG. 37, a collimator lens 11, a non-polarizing beam splitter 12, acylindrical lens 113, an annular reflection grating 14, and a radialgrating 115 for the rotary encoder are arranged in the optical path of alight beam emitted from a semiconductor-laser light source 10, and aphotosensor 16 is disposed in the reflecting direction of thenon-polarizing beam splitter 12. When the pitch of the grating on theradial grating 115 is represented by P1, the pitch P2 of the annularreflection grating 14 is set so as to satisfy a relationship of P2=P1/2.

According to such a configuration, a light beam L emitted from thesemiconductor-laser light source 10 becomes a substantially parallellight beam by the collimator lens 11, and is projected onto the radialgrating 115 linearly with respect to the orientation of arrangement ofthe grid line or the tangential direction after passing through thenon-polarizing beam splitter 12, the cylindrical lens 113 and a centralportion of the annular reflection grating 14. Diffracted light beams L+and L− reflected from the radial grating 115 are projected onto theannular reflection grating 14 in the form of an ellipse. The light beamis linearly projected onto the radial grating 115 due to the lightcondensing characteristics of the cylindrical lens 113. Since the powerof the cylindrical lens 113 does not operate in an axial directionorthogonal to the projected light beam, the light beam having only theoriginal substantially parallel property is obtained.

FIGS. 38 and 39 are diagrams illustrating results of tracking of±first-order diffracted light beams by projecting a light beam having adiameter of 1 mm in the same optical system. The annular reflectiongrating 14 locally operates as a linear diffraction grating having thepitch of P2. Hence, the light beams emitted from the linearly condensedregion of the radial grating 115 and projected onto all portions on theannular reflection grating 14 are diffracted and returned to theoriginal optical path to be projected onto and diffracted by the radialgrating 115, and are returned to the non-polarizing beam splitter 12 ina superposed state. This is an effect peculiar to the annular reflectiongrating 14.

The resultant light beam is guided in a direction different from thesemiconductor-laser light source 10 by the beam splitter 12, and isdetected as an interference light beam by the photosensor 16. When±first-order diffracted light beams are used, the period of light anddark of interference equals four periods with respect to one pitch ofthe radial grating 115. As shown in FIGS. 38 and 39, a substantiallycircular light beam is obtained on the photosensor 16.

FIG. 40 is a diagram illustrating a result of calculating a shift of theoptical path when the diffraction angle changes due to a change in theoscillation wavelength of the semiconductor-laser light source 10 by

λ=10 nm caused, for example, by a change in the temperature environment.In this case, the irradiated position on the annular reflection grating14 is slightly shifted due to a change in the diffraction angle of thediffracted light by the radial grating 115. However, since the lightbeam is diffracted with an original diffraction angle by the annularreflection grating 14, the position of rediffraction by the radialgrating 115 and the state of the emitted light beam are invariable.Hence, the state of interference is stable.

FIG. 41 is a diagram illustrating a shift of the optical path when theposition of the radial grating 115 is shifted by

x=0.5 mm. Even if the gap of the radial grating 115 at the positionilluminated by the light beam is reduced, the position of rediffractionby the radial grating 115 and she state of the emitted light beam areinvariable. Hence, the state of interference is stable.

FIGS. 42 and 43 are diagrams, each illustrating a result of calculationwhen the detection-head unit is shifted with respect to the radialgrating 115 by

y=0.5 mm in a tangential direction. This case is equivalent to the casethat the radial grating is eccentric by 0.5 mm, and is also equivalentto an azimuth error in the arrangement of the scale when using a lineargrating. Even if the irradiated position is shifted, since the lightbeam is returned to the original optical path by the function of theannular reflection grating 14, the position of rediffraction by theradial grating 115 and the state of the emitted light beam areinvariable. FIGS. 42 and 43 illustrate the optical paths of a+first-order diffracted light beam and a −first-order diffracted lightbeam, respectively. Although the irradiated positions on the photosensor16 are slightly shifted in the vertical direction, these light beams aresubstantially parallel to each other, and the state of interference isstable. The above-described amount of shift of 0.5 mm is provided inorder to facilitate understanding of the result of calculation. Theamount of shift in the actual encoder is much smaller.

FIGS. 44 and 45 are diagrams, each illustrating a shift of the opticalpath when a tilt error of

θz=0.5 degrees is given with respect the orientation of arrangement ofthe grating for the angle of installation between the radial grating 115and the detection-head unit. The result of reading of the optical pathsof the ±first-order diffracted light beams shown in FIGS. 44 and 45indicates that, even if a small amount of tilt error is added, thedifference between the ±first-order diffracted light beams in theposition of rediffraction by the radial grating 115 and the state ofemitted light beams does not change. Hence, the state of interference isstable.

In FIGS. 44 and 45, the incident light beams on the photosensor 16 areshifted from the surface of the photosensor 16. However, the amount ofshift of 0.5 mm is provided in order to facilitate understanding of theresult of calculation. The amount of shift in the actual encoder is muchsmaller, as in the cases of FIGS. 42 and 43.

FIGS. 46 and 47 are diagrams, each illustrating a shift of the opticalpath when a tilt error of

θy=0.5 degrees is given with respect the orientation of arrangement ofthe grating for the angle of installation between the radial grating 115and the detection-head unit. The result of reading of the optical pathsof the ±first-order diffracted light beams in FIGS. 46 and 47 indicatesthat, even if a small amount of tilt error is added, the differencebetween the ±first-order diffracted light beams in the position ofrediffraction by the radial grating 115 and the state of emitted lightbeams does not change. Hence, the state of interference is stable, andthe irradiated position on the photosensor 16 is substantially notshifted.

As described above, by combining the annular reflection diffractiongrating 14 with linear projection of a light beam, it is possible tomake provision of a small-size and high-resolution encoder having alarge allowance in a mounting error to be compatible with detection of astable interference signal.

FIG. 48 is a perspective view illustrating an encoder according to afifteenth embodiment of the present invention, in which two-phasesignals are detected by adding a phase-difference-signal generationoptical system having a polarizing element. By replacing adiffraction-grating scale 15 in this linear encoder by a radial grating,a rotary encoder may also be provided.

In FIG. 48, a collimator lens 11, a non-polarizing beam splitter 12, acylindrical lens 113, an annular reflection grating 14, two polarizingplates 20S and 20P whose orientations of polarization are shifted by 90degrees from each other, and a diffraction-grating scale 15 are arrangedin the optical path of a light beam from a semiconductor-laser lightsource 10, serving as an coherent light source. A ¼-wavelength plate 21and a non-polarizing beam splitter 22 are arranged in a reflectingdirection of the non-polarizing beam splitter 12. A polarizing plate 23a and a photosensor 16 a are disposed in the reflecting direction of thenon-polarizing beam splitter 22, and a polarizing plate 23 b and aphotosensor 16 b are arranged in the transmitting direction of thenon-polarizing beam splitter 22.

According to the above-described configuration, a light beam from thesemiconductor-laser light source 10 passes through the collimator lens11 and the non-polarizing beam splitter 12, and is substantiallyperpendicularly projected onto the diffraction-grating scale 15 afterpassing through the cylindrical lens 113 and a centraltransmitting-window portion of the annular reflection grating 14.

A +first-order diffracted light beam reflected from thediffraction-grating scale 15 is emitted with a diffraction angle θ, isdiffracted and reflected to the original optical path by the annularreflection grating 14 provided at an upper portion, and is returned tothe non-polarizing beam splitter 12 by being subjected to +first-orderdiffraction by the diffraction-grating scale 15.

A −first-order diffracted light beam reflected from thediffraction-grating scale 15 is emitted in an opposite direction with adiffraction angle θ, is diffracted and reflected to the original opticalpath by the annular reflection grating 14, and is returned to thenon-polarizing beam splitter 12 by being subjected to −first-orderdiffraction by the diffraction-grating scale 15.

The light beam projected from the semiconductor-laser light source 10onto the diffraction-grating scale 15 has vertically and horizontallypolarized components. Although the orientations of polarization of the±first-order diffracted light beams propagated to the non-polarizingbeam splitter 12 are shifted by 90 degrees from each other and thewavefronts of these light beams are superposed, these light beams arenot light-and-dark light beams.

As a result, the two light beams reflected by the non-polarizing beamsplitter 12 pass through the ¼-wavelength plate 21 and are convertedinto a linearly polarized light beam whose orientation of polarizationchanges based on the phase difference between the two light beams. Theobtained light beam is divided into two light beams by a non-polarizingbeam splitter 22 provided behind the ¼-wavelength plate 21. Onlyspecific orientations of polarization are extracted by the polarizingplates 23 a and 23 b, and light/dark signals are sensed by thephotodetectors 16 a and 16 b. The phases (timings) of these light/darksignals are provided by shifts of the orientation of polarization of thepolarizing plates 23 a and 23 b. That is, by shifting the orientationsof polarization of the polarizing plates 23 a and 23 b by 45 degrees inopposite directions, the phase difference between the light-dark signalsis set to 90 degrees.

FIG. 49 illustrates a sixteenth embodiment of the present invention, inwhich a linear diffraction lens 114 (a linear Fresnel zone plate) isused as optical means for projecting a light beam obtained by linearlycondensing a light beam. That is, a linear diffraction lens 25 isintegrally formed at a central transmitting-window portion of an annularreflection grating 14. The linear diffraction lens 25 is patterned sothat the pitch of the grating gradually changes from the central portionto the peripheral portion, in order to provide the function of acylindrical lens. Two-phase signals are detected by disposing a crystaloptical element for generating a phase-difference signal.

FIG. 50 illustrates a seventeenth embodiment of the present invention,in which an annular transmission grating 214 is adopted instead of theannular reflection grating 14. Diffracted light beams are reflected by areflecting surface 215 provided immediately behind the annulartransmission grating 214, and are projected onto a diffraction-gratingscale 15. The pitch of the annular transmission grating 214 is set tothe same value as the pitch of the diffraction-grating scale 15.

In FIG. 50, a collimator lens 11, a non-polarizing beam splitter 12, acylindrical lens 13, the annular transmission grating 214 having thereflecting surface 215 immediately behind it, two polarizing plates 20Sand 20P whose orientations of polarization are shifted by 90 degreesfrom each other, and the diffraction-grating scale 15 are arranged inthe optical path of a semiconductor-laser light source 10, serving as acoherent light source. A ¼-wavelength plate 21 and a non-polarizing beamsplitter 22 are arranged in the reflecting direction of thenon-polarizing beam splitter 12. A polarizing plate 23 a and aphotosensor 16 a are disposed in the reflecting direction of thenon-polarizing beam splitter 22, and a polarizing plate 23 b and aphotosensor 16 b are arranged in the transmitting direction of thenon-polarizing beam splitter 22.

According to the above-described configuration, a light beam from thesemiconductor-laser light source 10 passes through the collimator lens11, the non-polarizing beam splitter 12 and a centraltransmitting-window portion of the annular transmission grating 214, andis substantially perpendicularly projected onto the diffraction-gratingscale 15 via the cylindrical lens 13.

A +first-order diffracted light beam reflected from thediffraction-grating scale 15 is emitted with a diffraction angle θ, isreturned to the original optical path at the reflecting surface 215immediately after being refracted by the annular transmission grating214 provided at an upper portion, and is returned to the non-polarizingbeam splitter 12 by being subjected to +first-order diffraction by thediffraction-grating scale 15.

A −first-order diffracted light beam reflected from thediffraction-grating scale 15 is emitted with a diffraction angle θ inthe opposite direction, is returned to the original optical path at thereflecting surface 215 immediately after being diffracted by the annulartransmitting grating 214, is again diffracted and deflected by theannular transmitting grating 214, and is returned to the non-polarizingbeam splitter 12 by being subjected to −first-order diffraction by thediffraction-grating scale 15.

The light beam projected from the semiconductor-laser light source 10onto the diffraction-grating scale 15 has a polarized component whoseorientation is 45 degrees with respect to the optical axes of thepolarizing plates 20S and 20P. Hence, the ±first-order diffracted lightbeams propagated to the non-polarizing beam splitter 12 are linearlypolarized light beams having planes of polarization orthogonal to eachother. When the two light beams are subjected to vector synthesis afterpassing through the ¼-wavelength plate 21, a linearly polarized lightbeam whose orientation of polarization changes based on the phasedifference between the two light beams is obtained. Accordingly,light/dark signals are obtained by extracting only specific orientationsof polarization of light beams obtained by dividing the linearlypolarized light beam by the polarizing plates 23 a and 23 b. The phases(timings) of these light/dark signals are provided by shifts of theorientation of polarization of the polarizing plates 23 a and 23 b.Hence, by shifting the orientations of polarization of the polarizingplates 23 a and 23 b by 45 degrees in opposite directions, the phasedifference between the light-dark signals is set to 90 degrees.

In FIGS. 48 and 50, phase-difference plates may be inserted instead ofthe polarizing plates 20S, 20P, 23 a and 23 b in order to obtain aphase-difference signal. Furthermore, in FIG. 50, these polarized-statechanging elements may be inserted between the annular transmissiongrating 214 and the reflecting surface 215.

In the above-described embodiments, partial modification may beperformed with respect to the following items.

(a) The cylindrical lens 113 is replaced by an optical element havingequivalent functions.

(b) The cylindrical lens 113, and the annular reflection grating 14 orthe annular transmission grating 214 are integrated. For example, theannular reflection grating 14 is formed on the plane side of thecylindrical lens 113.

(c) The functions of the cylindrical lens 13 and the collimator lens 11are replaced by the functions of toric lenses or hologram lenses.

(d) In the diffraction-grating scale 15 and the radial diffractiongrating or the annular reflection grating 214, diffracted light beamshaving a diffraction order other than the ±first-order diffracted lightbeams are used.

(e) The polarizing plates 20S, 20P, 23 a and 23 b are replaced byprisms, each having a polarizing film, or fine-grating patterns, servingas other elements having the equivalent functions.

(f) The phase-difference plates, i.e., the ¼-wavelength plate and the⅛-wavelength plates, are replaced by fine-structure patterns or otheranisotropic materials having functions equivalent to the functions of acrystal optical element, such as quartz or the like.

(g) The same effects are obtained by providing at least two phases for aphase-difference signal and setting the phase difference to a valueother than 90 degrees, and partially changing the arrangement ofpolarizing elements or phase-difference plates.

(h) In the above-described embodiments, the non-polarizing beamsplitters 12 and 22 are used in order to guide light beams to beprojected onto the diffraction-grating scale 15 and to guiderediffracted light beams to the photosensor 16, respectively. However,the light beams may be guided by using any other appropriate beamdividing/synthesizing means, such as diffraction gratings or the like,or by separating light beams by spatially shifting forward and backwardoptical paths, or by selectively reflecting only one of the light beamsand guiding the selected light beam to the photosensor 16.

(i) For example, by changing the order of arrangement of the collimatorlens 11, the non-polarizing beam splitter 12, the cylindrical lens 13,the annular reflection grating 14 and the annular transmission grating214, another optical arrangement is adopted in order to provide a systemof linearly condensing a light beam onto the diffraction-grating scale15.

In the above-described embodiments, for example, an element having areflecting film deposited in the vacuum on the back of a glass plateprocessed by etching or the like can be used as the annular reflectiongrating. Hence, an excellent environment resisting property is provided.

As described above, the optical encoder according to the presentinvention has the following effects by optimizing a state of projectionof a light beam onto a diffraction-grating scale or an annulardiffraction grating.

(1) The interfered wavefronts of the diffracted light beams tend tocoincide with each other, a flat light-dark pattern is obtained, and astable encoder signal having an excellent S/N ratio can be obtained.

(2) Since, for example, a plane optical element can be used as theannular reflection grating, the space of arrangement is not complicated,and a small encoder can be easily provided.

(3) Since variations in the wavelength of the light beam from the lightsource are corrected, an interference signal is stabilized.

(4) Since an alignment error is corrected, even an encoder in which adiffraction-grating scale and a detection head are separated can berelatively easily mounted.

(5) The size of a beam reflecting optical element is small and thenumber of components is small. Hence, by adding deflection means tolight-beam projection means, the degree of freedom in the method or thedirection of projection of a light beam onto a diffraction-grating scaleis increased, and a wider posture of application can be obtained.

(6) Since rediffracted light beam is guided to the photosensor withoutgreatly spreading, it is possible to perform detection with a small lossand an excellent S/N ratio.

The individual components shown in outline in the drawings are all wellknown in the optical encoder arts and their specific construction andoperation are not critical to the operation or the best mode forcarrying out the invention.

While the present invention has been described with respect to what arepresently considered to be the preferred embodiments, it is to beunderstood that the invention is not limited to the disclosedembodiments. To the contrary, the present invention is intended to covervarious modifications and equivalent arrangements included within thespirit and scope of the appended claims. The scope of the followingclaims is to be accorded the broadest interpretation so as to encompassall such modifications and equivalent structures and functions.

1. A grating interference encoder comprising: an illuminating opticalsystem; a scale with a diffraction grating for generating two diffractedlight beams having different orders by being irradiated by a coherentlight beam from said illuminating optical system, said scale beingmovable relative to said illuminating optical system; an annular gratingfor deflecting the two diffracted light beams having the differentorders generated from said diffraction grating to cause the deflectedlight beams to be reprojected onto said diffraction grating; a condenserconfigured so that the diffracted light beams generated by saiddiffraction grating are condensed on said annular grating; aphotosensor; and a beam splitter for guiding a light beam, obtained byinterfering the rediffracted light beams with each other, to saidphotosensor.
 2. An encoder according to claim 1, wherein said annulargrating comprises a reflection diffraction grating.
 3. An encoderaccording to claim 1, wherein said annular grating is local.
 4. Anencoder according to claim 1, wherein said condenser comprises adiffraction lens.
 5. A grating interference encoder comprising: anilluminating optical system; a scale with a diffraction grating forgenerating two diffracted light beams having different orders by beingirradiated by a coherent light beam from said illuminating opticalsystem, said scale being movable relative to said illuminating opticalsystem; an annular grating for deflecting the two diffracted light beamshaving the different orders generated from said diffraction grating tocause the deflected light beams to be reprojected onto said diffractiongrating; a condenser configured so that a beam projected on thediffraction grating is condensed on said diffraction grating; aphotosensor; and a beam splitter for guiding a light beam, obtained byinterfering the rediffracted light beams with each other, to saidphotosensor.
 6. An encoder according to claim 5, wherein said annulargrating comprises a reflection diffraction grating.
 7. An encoderaccording to claim 5, wherein said annular grating is local.